1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system which transports optical signals over a dispersion-managed transmission line by using wavelength-division multiplexing (WDM) techniques.
2. Description of the Related Art
Long-haul optical transmission systems use in-line repeaters for retiming, reshaping, and regenerating information signals at appropriate intervals along the fiber-optic transmission path. While earlier regenerative repeaters convert optical signals back to electrical form before performing such signal processing functions, today's mainstream systems employ a linear, optical fiber amplifier that amplifies light signals purely in the optical domain. The use of optical amplifiers in place of regenerative circuits drastically reduces the number of components required in repeater equipment, resulting in a significant cost reduction and reliability improvement.
Recent years have also seen great leaps in the capacity of optical networks, mainly because of the development of wavelength-division multiplexing (WDM) technologies. WDM systems transmit many signals simultaneously on a single fiber, using different optical wavelengths. The combined use of WDM and optical amplification techniques has made it possible to send and receive large amounts of data over a great distance in the most economical way. In such modern optical transmission systems, however, chromatic dispersion and nonlinear effects in optical fibers become increasingly problematic, as the communications market dictates higher optical signal power, longer transmission distances, and denser wavelength channels. How to deal with chromatic dispersion and nonlinear effects is a key issue in designing a high-speed high-capacity optical transmission line.
Chromatic dispersion is a phenomenon that causes the separation of a light wave into its spectral components and thus broadens the pulse width as it propagates through a fiber medium. Nonlinear effects refer to a class of interferences that a relatively strong light could encounter when it goes through a glass medium, where non-linearity of the medium comes to the surface because of its properties affected by the intensity of the light. We must suppress such chromatic dispersion and nonlinear effects to sufficiently low levels to realize distortion-free long-distance transport of optical pulses.
Chromatic dispersion can, in fact, be canceled by placing dispersion compensating fibers on the optical path at appropriate intervals. Such design techniques are called “dispersion management,” and it is known that a dispersion-managed transmission line not only prevents the propagating optical pulses from suffering dispersion distortion, but also alleviates nonlinear effects on them.
The existing dispersion compensation techniques include the use of a non-zero dispersion-shifted fiber (NZ-DSF) in combination with a single mode fiber (SMF). More specifically, it is proposed to use. an NZ-DSF medium with a zero-dispersion wavelength of 1585 nm and a dispersion coefficient of about −2 ps/nm/km in the operating wavelength band, and an SMF medium with a zero-dispersion wavelength of 1310 nm and a dispersion coefficient of about −18 ps/nm/km in the same band. This technique is referred to herein as the “first conventional scheme.” For more details, see N. S. Bergano, “Wavelength Division Multiplexing in Long-Haul Transmission Systems,” IEEE Journal of Lightwave Technology, Vol. 14, No. 6, 1996, pp. 1299-1308.
The above first conventional scheme, however, has a disadvantage in its characteristics of dispersion slope (or first derivative of chromatic dispersion with respect to wavelength). That is, the combination of NZ-DSF and SMF does not allow us to expand the operating wavelength band for higher link capacity, because its minimum-dispersion window is too narrow to cancel dispersions for all the required wavelengths.
Another existing technique uses positive-dispersion fiber (+D fiber) and negative-dispersion fiber (−D fiber) to form each repeater section. Positive-dispersion fiber exhibits a positive dispersion in the operating wavelength band and zero dispersion at the wavelength of 1.3 μm. To cancel out the chromatic dispersion and dispersion slope of this +D fiber, a negative-dispersion fiber with opposite characteristics is combined. Each repeater section has a hybrid structure of half positive and half negative dispersions. This technique is referred to herein as the “second conventional scheme.” For more details, see M. Murakami et al., “Long-haul 16×10 WDM transmission experiment using higher order fiber dispersion management technique”, ECOC'98, 1998, pp. 313-314.
The second conventional scheme permits us to expand the signal wavelength band, since its dispersion compensation capability covers a wider range of optical wavelengths. The problem is, however, that WDM optical pulses (or bits) with different wavelengths are aligned at every boundary point between dispersion compensation intervals. This causes nonlinear effects in fibers, thus deforming transmission signals.
Meanwhile, Raman amplifiers are of particular interest in these years. Raman amplifiers are based on a physical phenomenon, known as the “Raman effect,” that the wavelength of light changes when a light beam is deflected by vibrating molecules. Signal amplification occurs if optical pump waves with the correct wavelength and power level are launched into the optical fiber, turning the full transmission length into an amplifying medium. The peak gain of Raman amplification is obtained at the wavelength that is about 100 nm longer than the pump light beam's. That is, the launched pump beam boosts optical signals with 100-nm longer wavelengths. This means that, for example, a 1.45-μm pump light beam is used to amplify 1.55-μm optical signals.
Compared to Erbium(Er3+)-doped fiber (EDF) amplifiers, the above-described Raman amplifiers are more suitable for optical repeaters for use in long-distance applications, because they allow the use of longer fiber cables to extend repeater intervals. Also, Raman amplifiers operate at low noise levels, as well as is applicable to wideband transmission when used with multiple-wavelength pump light sources.
The gain of a Raman amplifier depends on the length of an optical fiber serving as the amplification medium. This nature of Raman amplifiers poses a problem in constructing hybrid transmission lines using +D fiber and −D fiber. More specifically, +D and −D fibers are combined at an appropriate ratio that is determined by the desired dispersion characteristic of upstream or downstream repeater sections. Upstream sections may be designed to have a different length ratio from downstream sections in this case, and that difference in lengths could result in an unbalanced Raman gain distribution between the upstream and downstream transmission lines, which leads to reduced reliability of optical amplification.